It has been developed various method for manufacturing solid and semisolid polymers from hydrocarbons, for example 1-olefins. In one such method olefins, like ethylene, propylene, butene and pentenes are polymerized in the presence of catalysts in hydrocarbon diluents or in reaction medium formed by monomers. Hereby the reactants are kept in liquid phase or in supercritical pressure by maintaining a sufficient pressure in the polymerization reactor. When the forming polymer is insoluble or slightly soluble in said medium, the polymers forms as particles and both the reaction medium moving in the reactor and the product flow comprise a suspension formed by polymer particles, medium and monomers. The product flow is introduced usually to a separation vessel of polymer, where solids and liquid and gaseous substances are separated from each other.
One reactor type applied such methods is a tube reactor forming a continuous loop, where polymerization takes place in the turbulent flow circulating in the loop reactor. Polymer, diluents and monomers are taken out from the loop reactor either continuously or periodically via a discharge valve and are introduced into a separator, where polymer is separated by lowering pressure.
In order to control the polymerization reaction samples can be taken from the product flow of the reactor either continuously or periodically. The typical way is to take a sample from a gas flow coming out from the polymer separation tank and to analyze this gas sample by various methods, for example by gas chromatography. Such an arrangement is disclosed for example in U.S. Pat. No. 3,556,730.
In this known procedure, the period of delay that occurs from the departure of the product from the polymerization reactor until the time of start of the analysis is often considerably long and during that period essential chances may take place in the process. Thus the sample is not representative. That is why it would be desirable to make said period of delay in the analysis shorter.
In Finnish patent 85191, a method is disclosed by which essential shortening of the sampling delay is achieved. In this method, the sample is taken from the product pipe via an on/off-type shut valve, which is closed for the time of pressure surge produced on opening the discharge valve, and which valve is opened after pressure surge.
The control of polymer properties in slurry polymerization requires a precise control of concentrations. The control is carried out conventionally by analyzing feed concentrations or concentrations of gas phase separated from the product. These measurements do not give precise information from liquid phase concentrations in the reactor at certain moment.
When feed concentrations are analyzed, the real concentrations in the reactor are not known. When gas phase concentrations in a stirred-tank reactor are analyzed, the real liquid phase concentrations are not known. Further, problems are produced by the adhering of polymer particles in sample take-out system. When sample take-out takes place from the gas flow after the product pipe of the reactor (after the separation tank), the adhering of polymer particles in the sampling line causes problems. Further, possible return-blow gases in the product filters can disturb the analysis and there may be a long time delay until the concentration measurement has been carried out. Additionally, the analyzing from the gas phase is impossible, if the polymerization suspension is fed from one reactor to another without separation of gases. Analyzing straight from the slurry by using a conventional filter is not possible, because the filter is under reaction conditions and, if small catalyst particles remains on the face of the filter, said particles continue polymerization on the filter and plug it rapidly.
In Finnish patent application FI932159. a method for taking sample straight from the liquid phase of a loop reactor is disclosed. An in-line filter placed in the flow pipe attached to a loop reactor is applied in the method. The one end of said flow pipe is connected to the suction side of loop circulation pump and the other end is connected to the pressure side of the pump. The pump causes a high-speed flow, by which the filter surface inside the pipe is tried to keep clean.
In the method according this publication, the sample is taken out from the product suspension circulating in loop reactor and thereby the samples obtained fulfill the requirements of representatively. However, the method may not be applied, for example, in the sample take-out between two successive loop reactors. The in-line filter applied in the method is placed inside of a separate pipe, whereby the maintenance may be troublesome. The method is further based on that the flow velocity over the filter element must be considerably large in order to avoid plugging. In order to achieve a large flow velocity, the pressure difference over the loop pump must be sufficient. This limits the use of the method in smaller reactor. Further in certain operation conditions the flow velocity in the loop reactor may, however, be relatively low, which makes the available pressure difference lower. Likewise, under certain conditions the product suspension circulating in the loop reactor may contain relatively much of solids, whereby its circulating via a separate pipe may cause problems. Thus a need exist for further improvements in the sample take-out straight from the reaction medium.